Recent intense and rapid improvement of image quality in liquid crystal display devices has identified new technical challenges. One such challenge is local image unevenness in a display panel primarily due to the extremely fine pixel pitch pattern. This particular issue differs from overall unevenness of image quality in a display panel, also known as “Mura”, but is dependent on fundamental physical properties in terms of fringe field influence and the dielectric response of the liquid crystal material. For several years, so called “in-plane switching” (IPS) and fringe field switching (FFS) have become mainstream technologies for their relatively fast inter-gray shade optical response times as well as wide viewing angles. Despite such attractive features of IPS and ITS LCD technologies, both technologies have an image degradation problem with fine pixel pitch pattern displays. There are multiple interpretations of this problem, including flexo-electric effect (SID (Society for Information Display) Technical Digest, Paper No. 24.2: Investigation of Flexoelectric Effect in Vertically aligned in Plane Switching Mode by Low Frequency Driving, Cheng-Wwi Lai, Sau-Wen Tsao, Cho-Yan Chen, Tien-Lun Ting, Wen-Hao Hsu and Jenn-Jia Su, page 312-313 (2014)), distorted electric field influence on flickering (SID Technical Digest, Paper No. 733: Elimination of Image Flicker in a FFS Panel under Low Frequency Driving, Jung-Wook Kim, Tae-Hoon Choi, Tae-Hoon Yoon, E-Joon Choi and Ji-Hoon Lee, page 1081-1083 (2015)), and distorted electric field influence on image sticking (SID Technical Digest, Paper No. 49.2: Image Sticking Reduction of Fringe Field Switching LCDs, Darning Xu, Fenglin Peng, Haiwei Chen, Jiamin Yuan, Shin-Tson Wu, Ming-Chen Lii, Seok-Lyul Lee and Wen-Ching Tsai, page 739-742 (2015)). Aside from the particular technical accuracy of these analyses, there is consensus that the image quality degradation problem is due to the extremely fine pixel pitch of display devices.
The most advanced recent fine pixel pitch FFS LCD panels, used widely in smart phones, have approximately 800 pixels per inch, or a sub-pixel pitch of approximately 30 microns, unlike displays with large pixel pitches of more than 200 microns. Panel with such fine pixel pitches must overcome the local fringe field influence, which degrades local image quality in a display panel. Although the image degradation of each pixel pitch is local issue, the ratio of the degradation area to overall area increases as the PPI (Pixel Per Inch) increases, due to the fine pixel pitch. Therefore, the image degradation issue becomes an overall display issue, rather than a local area issue. Moreover, even finer pixel pitch is required to provide more vivid image quality. Image degradation due to increased pixel pitch is accompanied by slower response. Smaller pixel pitch results in increased boundary area relative to the bulk response area. Due to the larger boundary area at finer pixel pitches, the conflict area of liquid crystal molecular movement becomes larger, resulting in slower response time. Such an increase in response time is a critical issue, not only for display applications, but also for phase modulation applications. Therefore, local image degradation with fine pixel pitch is a technical problem in need of solution.
In addition to high resolution display applications, such as smart phones, an extremely fine pixel pitch pattern is also required for phase modulation purposes such as three-dimension display applications and light beam steering. Such applications require control of light phase rather than light amplitude. Therefore, the requirement for extremely fine pixel pitch is intrinsic. It is increasingly important to solve local liquid crystal molecular behavior under electric field application for both amplitude modulation, primarily used for display applications, and phase modulation, primarily used for photonics applications. Additionally, both amplitude and phase modulation applications require very fast optical response. To satisfy such technical requirements, sufficiently fast optical response and sufficient stability to retain the initial designed alignment configuration to resist disturbing the electric field, such as uncontrollable fringe field, are both critical issues.
Current conventional IPS/FFS LCD technology requires “inter digit electrode structure” to drive liquid crystal molecules. Typical inter digit electrode structure is described in SID (Society for Information Display) Technical Digest, Paper No. 44.3: 513-ppi Liquid Crystal Display Using C-Axis Aligned Crystalline Oxide Semiconductor with Narrow Bezel and Aperture Ratio Greater than 50%, Kouhei Toyotaka and others, page 634-637 (2014). When the pixel pitch of an inter digit electrode pitch represented by so called line and space (the sum of electrode line width and electrode gap) is larger than approximately 200 microns, the fringe electric field has no significant effect on liquid crystal molecule alignment. Due to the relatively large distance between neighboring electrodes relative to electrode length, most of the electric field applied between two neighboring inter digit electrodes is substantially parallel to the substrates, resulting in a substantially ideal electric field direction to drive IPS/FFS liquid crystal molecules. Conversely, when the pixel pitch is smaller than approximately 50 microns, an effective electric field to drive liquid crystal molecules in an IPS/FFS LCD panel significantly distorts the electric field direction as shown in FIG. 1. Due to the small distance between two neighboring electrodes, the electric field near each electrode edge area has significant distortion. The actual electric field distortion is even enhanced by the liquid crystal itself. The dielectric constant of most liquid crystal material is larger than that of air or in a vacuum, and often larger than 20. Since the liquid crystal panel is filled with a medium having a larger dielectric constant, the actual electric field power line is more distorted than in a vacuum or with air, as illustrated in FIG. 2 due to dielectric reordering or the liquid crystal molecules. Such a significantly distorted electric field drives liquid crystal molecules into almost vertical alignment when positive dielectric anisotropy liquid crystal material is deployed as an IPS/FFS LCD, as illustrated in FIG. 3. Due to the positive dielectric anisotropy of liquid crystal molecules (the dielectric constant parallel to the long molecular axis is larger than that perpendicular to the long molecular axis), the liquid crystal molecules align along the electric power line direction. When the electric power line is distorted, as shown in FIGS. 1 and 2, the liquid crystal molecules change their molecular direction with the electric power line direction. Such electric field distortion is determined not only by inter digit electrode pixel pitch, but also by the mutual relationship between pixel pitch and the liquid crystal panel gap, as well as the permittivity of the filled liquid crystal materials. Since IPS/FFS LCDs are driven by the fringe electric field, they work well without any significant degradation of image quality, including reduction of contrast ratio, flickering, and image sticking when the applied electric field direction is substantially parallel to the top and bottom substrates. However, when electrode pitch shrinks below approximately 50 microns, the applied electric fringe field becomes significantly distorted, resulting in unevenness of image quality as well as flickering and image sticking as described by the published references cited above, regardless of liquid crystal material.
Local electric field distortion creates another technical problem. The liquid crystal molecular response has variation due to uneven electric field strength. Such response variation creates mutual conflict among liquid crystal movement, resulting in reduced response time.
As described above, fringe electric field distortion is an intrinsic phenomenon governed by the relative distance between electrode gap and electrode width. Additionally, market demand for display devices and three-dimensional displays is trending toward fine pitch, higher resolution displays. Moreover, for phase modulation devices, most applications require extremely fine pixel pitch configuration to have sufficiently smooth continuous phase variation in spatially. Here, the meaning of smooth continuous phase variation in spatially is provided by a small difference in refractive indices formed by slightly different orientation of the liquid crystal molecules, along with fine pitch patterned electrode in a liquid crystal panel. Therefore, electric field distortion is a more significant issue for phase modulation devices than display devices in terms of the controllability of smooth and/or continuous refraction indices of liquid crystal panels. Therefore, new liquid crystal device technology which enables fine pixel pitch and high resolution devices without image quality degradation, including flickering, image sticking, and uniform image quality, is desired.
Electric power line distortion is also a technical issue for phase modulation devices. In particular, the problem exists for phase modulation devices when phase modulation switching mixes both in-plane and out-of-plane modulation. Some phase modulation applications require only in-plane retardation switching for clear phase modulation. Other applications require only out-of-phase retardation switching for clear phase modulation. IPS/FFS liquid crystal devices only require in-plane retardation switching. However, there is a mixture of in-plane and out-of-plane retardation due to the fringe field influence described above. In particular, for the fine pixel pitch configuration required for phase modulation devices, current IPS/FFS liquid crystal devices do not separate in-plane and out-of-plane retardation switching.
For out-of-plane retardation, a planar aligned electrically controlled birefringence (ECB) liquid crystal panel should provide only out-of-plane retardation switching. FIGS. 4(a) and 4(b) show a typical ECB panel operating principle. As shown in FIG. 4(a), when no voltage is applied, liquid crystal molecules show a planar aligned structure which is substantially parallel to the top and bottom substrates. The alignment is not completely parallel due to the surface pre-tilt angle. A typical surface pre-tilt angle is between 1 and 3 degrees. With such a small pre-tilt angle, when a voltage is applied to the panel, each liquid crystal molecule in the panel moves in the same direction. Such a small pre-tilt angle works as a direction leading trigger for each liquid crystal molecule. When an external driving voltage is applied, as shown in FIG. 4(b), most liquid crystal molecules align perpendicularly to the top and bottom substrates, or along the electric power line. Therefore, the ECB case illustrated in FIGS. 4(a) and 4(b) provides only out-of-plane retardation switching between with and without application of voltages to the panels. The cases shown in FIGS. 4(a) and 4(b) use large, single electrodes. Therefore, the fringe field influence is negligible. When the fringe field is negligible, such an ECB panel provides only out-of-plane retardation switching. However, fine pixel pitch is required in most cases. In such case, the fringe field influence is not negligible, but a large electric field, as illustrated in FIG. 5. As shown in FIG. 5, fine pixel pitch creates a fringe field and liquid crystal molecules do not respond with simple parallel or perpendicular alignment to the top and bottom substrates in the distorted electric power line area, resulting in a mixture of both in-plane and out-of-plane retardation switching. Therefore, for both amplitude and phase modulation purposes, the fringe field influence creates unacceptable levels of degradation when the pixel pitch size is less than approximately 50 microns.